Excepting the fact that you are living in USA and wish to tour France, Switzerland, Austria, Italy and Germany with your partner in June-July, your inquiry is very broad and whether you have 30 days vacation or 60 days vacation cannot be assumed. Therefore we can only point to where to gather more information to make your calculations and committment.
Bike hire; hire duration and hire cost; is your deciding factor. Flights are cheap nowadays, and accomodation in those countries is very good value for money. Check bike hire prices
If you have 15 or so bikers in a group, then call to Mike Mandell at Motorcycle Express to have your bikes and helmets, etc, containered as one lot into Europe, and back again of course. His contact details are tel: +1 (516) 682-9220
Possibly he may quote you consolidation services for a single bike.
Hiring individually is expensive. See Beach's Motorcycle Adventures who will give you BMW bike hire prices in Germany, Italy, etc.
However the easy way to do it taking in all the countries stated, including bike hire and airport pick-up at Munich Airport, is to use Edelweiss Bike Tours. Their website is listed under with other Tour Packagers on this site. They do a 2-tour back-to-back package that allows you to keep the bike between tours if you have a month or more to sojourn!
From UK you can hire starting from around 450 USD per week, inclusive of roadside recovery and Insurance. Inevitably extras pop up for "foreign" travel within the continent, luggage, etc,. Gear hire is also available. A deposit of around 800 USD may be requested. Add to this 300 USD for getting across to France and back by ferry boat. 35 USD for gas per day and 80 USD per day for the pair of you food and sleepwise. If you go this route then be prepared to fax in advance copies of your passport, driving license and motoring offences (if any) so that you can be cleared by both the Hirer, and the Insurer before paying out a deposit. Note, a minimum age of 25 or 30 years may be required.
If we can provide local backup to help you get your own bike over to UK then let us know - it is the best way for an extended trip. June - July is perfect weatherwise enabling you to wear full protections throughout the tour and not too hot. The odd rain shower can be expected. 300 miles per day (mpd) tells me you ride like myself. Note: it is hard going via the Alps, Dolomites, etc, with altitudes up to 2,800 metres (9,000 feet) and 250 mpd can be quite tiring. Nevertheless a sauna will get you fully recovered for the following day. Allow yourself time to take photos...... Your choice of countries offers superb riding. I hope your passenger enjoys the challenges too.
Stilfser Jock 2340 meters above sea level (Passo Stelvio) near Bormio. An awesome collection of 74 hairpin bends requiring advanced riding ability. Beware dropping your bike if you miss a gear during accent, and dress well for the temperature change top to bottom.
Cycle Search Intl (containering 10 to 15 bikes to Europe)
Beach's Motorcycle Adventures (hire in US and collect in Germany)